FIG. 1 schematically shows an optical modulator according to the principle of the Mach-Zehnder interferometer, commonly called MZI modulator. The modulator includes an optical waveguide receiving a power P, which is divided into two branches at a point S. The two branches join again at a point J, one directly and the other via an electro-optical phase shifter 10. Each branch carries half of the original optical power.
The carriers arriving via the two branches are summed at point J of the modulator, one carrier having been shifted by φ by the phase shifter 10. The resulting carrier has a power P·cos 2(φ/2), neglecting the optical losses.
FIG. 2 shows a perspective view of a branch of the waveguide 12 incorporating a phase shifter 10, shown in gray. The waveguide is formed in a transparent island, of intrinsic semiconductor material, having an inverted “T” cross-section, whose central rib WG carries the optical beam. The phase shifter is configured to replace a segment of the waveguide and has the same inverted “T” cross-section. The edges of the phase shifter bear electrical contacts serving to control the phase shifter—they usually extend above the plane of the waveguide, as shown, to reach the metal levels.
FIG. 3 schematically shows a sectional view of a so-called high-speed phase modulator (HSPM) phase shifter 10. The cross-section plane is perpendicular to the axis of the optical waveguide. A dashed circle, at the center of rib WG represents the area of concentration of the optical beam.
The phase shifter includes a semiconductor structure of same nature as that of the waveguide, generally silicon, forming a PN junction 14 in a plane parallel to the axis of the waveguide and offset with respect to the axis. The junction 14 has been shown, as an example, at the right sidewall of the rib WG, but its position may vary, according to the application, between the sidewall and the center of the rib.
A P-doped region extends to the left of the junction 14, conforming to the section of the waveguide, i.e. including an elevated portion at the level of the rib WG, and a lower side wing 16 towards the left edge. Zone P ends to the left by a P+ doped raised area bearing an anode contact A. An N− doped wing 18 extends to the right of the junction 14, conforming to the section of the waveguide. The wing 18 ends to the right by an N+ doped raised area bearing a cathode contact C. The structure of the phase shifter may be formed on an insulating substrate, for example a buried oxide layer BOX.
To control the phase shifter of FIG. 3, a voltage is applied between the anode and cathode contacts A, C, which reverse-biases the junction 14 (the “plus” on the cathode and the “minus” on the anode). This configuration causes a displacement of electrons e from the N region to the cathode and of holes h from the P region to the anode, and the creation of a depletion region D in the vicinity of the junction 14. The carrier concentration is thus modified in accordance with the magnitude of the bias voltage in the area crossed by the optical beam, which results in a corresponding modification of the refractive index of this area.
The sensitivity of the electro-optical phase shifter depends on the capacitance CJ of the junction 14, which represents the carrier concentration that is obtainable in the central area WG as a function of the control voltage. The sensitivity increases with the doping level of the area WG, but increasing the doping level also increases the optical losses. The P doping level in this area is often higher than the doping level of the original substrate to reach a satisfactory sensitivity.